4 THE GLACIAL LAKE AGASSIZ. 



the interior portion of the continent thence to Ottawa and Montreal. It is 

 evident, therefore, that no barrier of land held the lake which covered the 

 Red River Valley and a large part of Manitoba. But a southward outlet 

 is found through which the lake flowed to the Mississippi, and no marine 

 fossils have been detected in or above the drift upon this area, from which 

 reasons it is certain that these ancient shore-lines were not produced by 

 subsidence of the land beneath the sea-level, followed by reelevation. 

 While the beaches and deltas here described are thus known to be 

 lacustrine, fossils have been discovered in them at only two localities, these 

 being shells of five species of fresh-water moUusks, occumng in beaches 

 that belong to the middle and later part of the lake's history. 



From all these features of the former lake, when they are considered 

 in their relationship to the Grlacial period and its drift deposits, we are led to 

 the conclusion that the northern barrier by which its water was held in was 

 a waning ice-sheet. The glacial striae, till, tenninal moraines, and other drift 

 formations prove that the northern half of our continent has been enveloped 

 by a continuous mantle of ice stretching- from the eastern shores of New 

 England and Canada west to the Rocky Mountains and the Pacific, and 

 from the northern part of the United States to the Arctic Sea. When this 

 ice-sheet was melted away, its border gradually withdrew from south to 

 north, and hydrographic basins descending northward were temporarily 

 occupied by glacial lakes, held by the ice barrier until its continued 

 retreat allowed free drainage of these basins in the direction of their slopes. 

 This explanation fully accounts for the presence of Lake Agassiz in the 

 basin of the Red River and of Lake Winnipeg during the recession of the 

 ice-sheet, for the scantiness of its fauna, and for the northward ascent of its 

 originally level shores, since the earth's crust had been depressed by the ice 

 burden, and was uplifted, in the preservation of its equilibrium, when the 

 ice-sheet departed. 



The work of Gilbert, Chamberlin, Leverett, and others in the basins 

 of the Laurentian lakes has proved that their formerly much higher levels, 

 marked by shore-lines similar to those of Lake Agassiz, were contempora- 

 neous with the departure of the ice-sheet and the formation of its recessional 

 moraines. Records of many other smaller glacial lakes have been observed 



